1. Field of the Invention
This invention relates to gas turbine engines, and more particularly to a seal member for a coolant cavity in the turbine case of such an engine.
2. Description of the Prior Art
A gas turbine engine has a compression section, a combustion section and a turbine section. A rotor extends axially through the turbine section. A row of rotor blades extend outwardly from the rotor. A stator circumscribes the rotor. The stator includes an engine case, an outer air seal and rows of stator vanes. The engine case positions and supports the outer seal and the stator vanes. The outer air seal is radially spaced from the row of rotor blades leaving a tip clearance therebetween. Working medium gases are pressurized in the compression section, burned with fuel in the combustion section and expanded in the turbine section. The temperature of the working medium gases discharging from the combustion section into the turbine often exceeds fourteen hundred degrees Celsius (1400.degree. C.).
The hot gases entering the turbine section lose heat to the turbine blades and the case. Uncontrolled heating of the case may decrease engine efficiency as the outer air seals are displaced outwardly with increasing case diameter.
In modern engines, cooling air is flowed through passages and cooling air chambers on the interior of the case to remove heat from the case. The case forms the outer wall of each cooling chamber. Each cooling air chamber has an inner wall to suppress heat transfer to the case. The inner wall, having an inner surface facing the hot gases, shields a portion of the outer case from radiation. The inner wall functions to block both the hot gases from contacting a portion of the outer case and the cooling air from flowing into the hot gas path.
U.S. Pat. No. 3,730,640 to Rice et al. entitled "Sealed Ring for Gas Turbine" is representative of structures in which the outer surface of a ring cooperates with the inner surface of a case to form a cooling cavity. The ring forms an inner wall having a "Z" shape. In Rice et al., cooling air flows between a flange on the Z-ring and a flange on the case to enter the cooling cavity. The cooling air flows out of the cooling cavity to a downstream location through holes in a downstream flange. Rice et al. is not especially concerned with cooling air leakage through the inner wall. Cooling air flows through the Z-ring to a cooling air cavity formed by the outer surface of the Z-ring and the inner surface of a vane platform. Rice et al. does not attempt to form an airtight seal between the ring and the upstream flange and the ring and the downstream flange.
In U.S. Pat. No. 3,992,126 to Brown et al. entitled "Turbine Cooling", an annular air cavity is formed by a deformable ring and the outer case. The deformable ring acts as the inner wall. Cooling air exits from the chamber through orifices in the ring underneath the vane platform and between adjacent vanes. The ring engages an upstream flange extending inwardly from the case and a downstream flange extending inwardly from the case.
In addition to heating the case, the hot gases cause another problem. As the working medium gases expand through the turbine section, the gases exert nonuniform aerodynamic forces on the vanes. These forces are the primary cause of vane vibration during high speed turbomachine operation. The vibrations and the forces can create high stresses in the vanes which ultimately may cause fatigue failure.
Many engines use vibration dampers to dampen these vibrations. U.S. Pat. No. 3,326,523 to Bobo entitled "Stator Vane Assembly Having Composite Sectors", is representative of structures in which a damping means engages a base of a vane to provide vibration damping. The invention in Bobo employs a spring member mounted in an arcuate groove having a dovetail shape. U.S. Pat. No. 3,730,640 to Rice et al. employs a circumferentially extending spring to position and dampen a circumferentially extending ring serving as an outer air seal. The use of vibration dampers enhances the fatigue life of the vanes.
Although the fatigue life, the creep resistance and the performance of the turbine case are enhanced by using cooling air to reduce case temperatures, there is a penalty. The increased performance of the engine resulting from the improved blade tip clearance control is diminished by the use of cooling air. Accordingly, scientists and engineers are working to design a seal ring for cooling air chambers having increased sealing effectiveness, an adequate fatigue life, and an ability to dampen vane vibration.